Doping of bulk semiconductors, the process of intentional introduction of impurity atoms into a crystal discovered back in the 1940s, is a key route for tuning their properties. Its introduction allowed the wide-spread application of semiconductors in electronic and electro-optic components [1]. Controlling the size and dimensionality of semiconductor structures is an additional powerful way to tune their properties via quantum confinement effects. In this respect, colloidal semiconductor nanocrystals have emerged as a family of materials with size dependent optical and electronic properties. Combined with their capability for wet-chemical processing, this has led to nanocrystal-based light emitting diodes, solar cells and transistor devices prepared via facile and scalable bottom-up approaches. Impurity doping in such colloidal nanocrystals still remains an open challenge [2]. From the synthesis side, the introduction of a few impurity atoms into a nanocrystal which contains only a few hundred atoms may lead to their expulsion to the surface [3-5] or compromise the crystal structure. This inherently creates a highly doped nanocrystal under strong quantum confinement, and the electronic and optical properties in such circumstances are still unresolved.
Several strategies have been employed so far for doping nanocrystals. Binding ligands on the nanoparticle surface, which can donate carriers, or electrochemical carrier injection, have been shown to yield n-type doping in semiconductor nanocrystal superlattices [6-8]. While of great interest, such remote doping differs from substitutional doping, which has been studied mainly for color center impurities [9] and magnetic impurities, notably Mn atoms [10,11], providing insight to the challenging chemistry [12]. It should be noted, that despite efforts to concentrate such doping solely in the nanocrystal, significant amounts of the dopant materials were found associated with the nanocrystals surface.
Introduction of dopant precursors at specific stages of nanoparticle growth were effective in controlling the impurity location [13]. More recently, some progress has been made towards producing n-type CdSe quantum-dots (QDs) using tin and indium impurities [14, 15], and p-type InP using Cu impurities [16].